The brake light mystery

[Simpo Two]

When I bought my 2007 DB9 3 months ago it had water droplets in the NSR lamp. 'They all do that' said the dealer.

Well in fact the car had had a prang in that corner and the unit was either repaired or a s/h replacement. Not long after purchase the brake lamp warning light came on. That's annoying because it wipes out all the data you get in the section. My local place got it dried out and re-fixed, but it failed again the next day. An Aston specialist said it was beyond repair. So I resolved to fix the bd myself. Out it came, prised apart, thoroughly dried out in the airing cupboard then reassembled with clear sealant and clamps all round, insulating tape all round the cables and connector, and a big chunk of bubblewrap stuffed into the gap on the left to stop mud and water being thrown all over the back of the unit from the back wheel like it was before. No way is water getting in again.

A few days later and the fking warning light comes on again. The first time it came on when going over a bump. The lamp was dry inside so I deduced the problem.was now a loose connection somewhere. But then a pattern developed. The light would be fine for an outward trip, then fail on the way home. Today it did it again - fine on the way out, failed 30 secs into the return journey.

But hold on, what's the other difference? The journey out is in daylight, the homeward journey after dark. What else is on after dark? Why, the headlights of course. So I get home, turn the engine, ignition off, and turn the headlight switch off. Turn the ignition on again and hurrah, it's happy and my digital speedo is back. I put the headlights on - and instantly the brake warning light comes on again.

So the brake warning light is linked to the headlights. A voltage drop perhaps. What's the fix? The battery seems to be in fine condition.

[bullet7]

Do the brake lights actually work when the warning lamp is on? Sounds as though it might be an earthing issue, although I must say I have not seen this very issue before.

[paulrog1]

These electrical gremilins could be due to the tracker, look at BR youtube vids and get it disconnected.

[Simpo Two]

bullet7 said:

Do the brake lights actually work when the warning lamp is on? Sounds as though it might be an earthing issue, although I must say I have not seen this very issue before.

Thanks for the reply. I'm not sure. They both work when the warning light is out so the unit is OK. I'll have to make a point of trying them when I get home after dark next time.

Could an earthing problem become noticeable only when the car is fully warm? (ie after 20-30 mins running?) If so it might be contributing to another issue.

paulrog1 said:

These electrical gremilins could be due to the tracker, look at BR youtube vids and get it disconnected.

Interesting; the car would have had a Tracker from new but the subscription has lapsed and AFAIK is inactive. But could it still be influencing something?

[bullet7]

The tracker does still have an impact on the vehicle system even though you are not actively using it. Bamford Rose have a YouTube video about it if I remember correctly. You should get it disabled (I had mine done at last service) because they do cause odd electrical issues when they go wrong. It might be this, although I would have thought you might have seen other symptoms. Commonly it causes incorrect error messages/lights on the dashboard and draining of the battery.

[Simpo Two]

bullet7 said:

The tracker does still have an impact on the vehicle system even though you are not actively using it. Bamford Rose have a YouTube video about it if I remember correctly. You should get it disabled (I had mine done at last service) because they do cause odd electrical issues when they go wrong. It might be this, although I would have thought you might have seen other symptoms. Commonly it causes incorrect error messages/lights on the dashboard and draining of the battery.

Thanks. Before diving in I need to do a more critical assessment of exactly when the light comes on and when it doesn't. Yesterday was clear cut - it came on immediately with the headlights. But I don't think the engine was running. At other times it's been more random and whilst driving. I need to keep some notes!

I'll ask the mechanic to do the tracker along with the other stuff when he's back at work; that will eliminate one variable.

[Simpo Two]

Today I had the chance to investigate this a bit further.

When driving without the headlights on everything seems fine. Then it gets dark, so I put the headlights on, and immediately the brake warning light comes on.

So I get home and turn off the engine, ignition and headlights. Turn the ignition back on - power beauty soul yada - system seems happy. But turn the headlights on and - bingo, the brake warning light comes on. As I can't operate the brake pedal and see the back of the car at the same time I tap up a neighbour for help. When the brake light warning is on, the left hand light is brighter than the right one.

Does that provide any clues?

[leman600]

Not sure if it's possible but you could rig it up temporarily so that the lights are swapped and see if the fault follows the light. (Check the plug pins are giving the same voltages each side for safety. Should be)
I think the tracker issue that's regularly mentioned possibly causes other problems by the fact that it discharges the battery to a point where all are cars exhibit some very strange symptoms. It's not necessarily directly the cause.
To delete this properly it is physically disconnected but also AMDS programmed out so the system isn't looking for it.

[Simpo Two]

Thanks Ian, all replies appreciated!

leman600 said:

Not sure if it's possible but you could rig it up temporarily so that the lights are swapped and see if the fault follows the light. (Check the plug pins are giving the same voltages each side for safety. Should be)

I'd really just like to pay a clever bloke with a multimeter to tell me what the problem is so it can be fixed.

leman600 said:

I think the tracker issue that's regularly mentioned possibly causes other problems by the fact that it discharges the battery to a point where all are cars exhibit some very strange symptoms. It's not necessarily directly the cause.
To delete this properly it is physically disconnected but also AMDS programmed out so the system isn't looking for it.

I think I'll pass on that then, another £1,000 only to find it wasn't that after all...

[N7GTX]

I'd start with the headlight that's not so bright. This could be something as simple as a poor earth. Disconnect the connector from the bulb and ensure terminals are clean and tight fitting. Without a multimeter it will be difficult to check accurately but attaching a wire to the earth wire at the connector and the other end to a good clean earth point nearby might show the problem if the light becomes brighter.
The only way to know for sure about the voltage at the connector is with a meter where you will get battery voltage with ignition on (12.6v approx) or charging voltage with the engine running (14v+ approx).

[Simpo Two]

N7GTX said:

I'd start with the headlight that's not so bright. This could be something as simple as a poor earth. Disconnect the connector from the bulb and ensure terminals are clean and tight fitting. Without a multimeter it will be difficult to check accurately but attaching a wire to the earth wire at the connector and the other end to a good clean earth point nearby might show the problem if the light becomes brighter.
The only way to know for sure about the voltage at the connector is with a meter where you will get battery voltage with ignition on (12.6v approx) or charging voltage with the engine running (14v+ approx).

Thanks Iain. It's the NS brake light not a headlight if that makes any difference to your plan.

It seems to me that the brake light and sidelight 'jam on' together - as if the brake light is always on when the sidelight is on. With the headlights off (by which I mean all lights), the brake light will work again even though the error message remains displayed.

As for disconnecting things, I know the rectangular connector that goes into the back of the unit, but I didn't dismantle anything more when I split it apart to dry it out. Are there other bulbs/connections I can get to? I really don't want to have to unglue the two halves again!

Would trying it with just sidelights (as opposed to dipped.main beam) help to find the fault?

[N7GTX]

Ah my mistake. Contact BBA Reman 01634 687222 who specialise in repairing all sorts of car electrics. They often guarantee repairs for life on many parts too.

Brake/sidelight issues are problems they know about and can fix/repair on DB9s.

http://www.bba-reman.com/gb/content/about_bba_rema...

[john ryan]

It is an earthing fault on one rear light, and one strong suspect is the in-line electrical connector

[Simpo Two]

N7GTX said:

Ah my mistake. Contact BBA Reman 01634 687222 who specialise in repairing all sorts of car electrics. They often guarantee repairs for life on many parts too.

Brake/sidelight issues are problems they know about and can fix/repair on DB9s.

http://www.bba-reman.com/gb/content/about_bba_rema...

Thanks for the info Iain, I'll keep that in mind!

john ryan said:

It is an earthing fault on one rear light, and one strong suspect is the in-line electrical connector

That sounds nice and definite, and I can get to that. Short of disconnecting it, checking the pins and refitting and wiggling it, is there anything else I can do to ensure a good connection? Some sort of spray perhaps? The connector is integral with the back of the unit so can't be replaced (as far as I can see).

[john ryan]

Side light/brake light earthing faults are very common. Typically when both are on, one brake light will extinguish, and the other will become brighter. You will need some electrical forensic skills to find the faulty connection, but any auto electrician will recognise the problem.

[leman600]

You could take a temporary earth to the light to confirm and then hunt for the actual poor earth.

[N7GTX]

In normal cars which have filament bulbs this would be an easy fix but this is an LED unit with solid state circuitry inside and which has probably suffered some damp/corrosion issue. The price of new lamp units is silly, hence why I mentioned BBA Reman.

Of course you should check the connectors and spray with Contact Cleaner to eliminate them first. Then if its possible, swap the rear units over temporarily by plugging them in and see if the problem moves to the other side. If then the fault moves across the problem is the unit itself. If the problem remains on the original side then its a supply or earth fault.

[Simpo Two]

Thanks guys. Having no idea how to take a temporary earth anywhere, and not wishing to blow anything up I've booked it into an automotive electrician.

And they say that 'BOAT' stands for 'break out another thousand'. Ha.

[Cold]

Buy a new light unit.

[Simpo Two]

Cold said:

Buy a new light unit.

It may come to that, but it's at the end of the flow chart.